Nonwoven fabrics are used in a wide variety of applications where the engineered qualities of the fabrics can be advantageously employed. The use of selected thermoplastic polymers in the construction of the fibrous fabric component, selected treatment of the fibrous component (either while in fibrous form or in an integrated structure), and selected use of various mechanisms by which the fibrous component is integrated into a useful fabric, are typical variables by which to adjust and alter the performance of the resultant nonwoven fabric.
In and of themselves, continuous filament fabrics are relatively highly porous, and ordinarily require an additional component in order to achieve the required barrier performance. Typically, barrier performance has been enhanced by the use of a barrier “melt-blown” layer of very fine filaments, which are drawn and fragmented by a high velocity air stream, and deposited into a self-annealing mass. Typically, such a melt-blown layer exhibits very low porosity, enhancing the barrier properties of composite fabrics formed with spunbond and melt-blown layers. Such nonwoven constructs have been utilized as battery separators as disclosed in U.S. Pat. No. 5,492,781, the disclosure of which is herein incorporated by reference.
A battery is a device that is capable of converting electrochemical energy into direct current and may be designated as either a primary or a secondary battery. The difference between primary batteries and secondary batteries is in the type of electrochemically active material that is employed. Primary batteries, or fuel cells, are defined as those battery systems that create electric current through the oxidation of fossil fuels and their derivatives. Kirk-Othmer Encyclopedia of Chemical Technology, 3, 545 et seq. (1978). As such, when the fuel is completely consumed, the life of the battery is completely exhausted. Secondary cells, on the other hand, generate electrical current through reversible chemical reactions, and thus may be recharged by applying an external current through the battery in a direction opposite to normal current flow. Id. at 569.
Batteries are made up of one or more battery cells. In its most elementary form, a battery cell comprises a pair of plates, namely, an anode and a cathode, a battery separator, and an electrolyte. When an electrical load is applied to the battery, electrons are generated through oxidation at the anode. The electrons thus generated pass through the load, then return to the battery cell at the cathode, where the cathode is reduced.
In such battery cells, the electrolytic solution, i.e., the solution containing the electrolyte, is the medium for mass transport between the plates. The primary functions of the battery separator are to prevent physical contact between the plates and to retain the electrolytic solution. In a starved-electrolyte battery cell, the separator completely occupies the space between the plates, and the electrolytic solution is completely contained within the battery separator. The battery separator thus functions as the reservoir for the electrolytic solution in such cells. The battery separator must be chemically inert so as to withstand the harsh chemical conditions found within battery cells.
Fluorocarbon polymers, preferably ethylene-fluorocarbon copolymers, particularly ethylene-chlorotrifluoroethylene (ECTFE) are known to have excellent chemical resistance and have been used in fuel cell applications. In addition, these polymers have been used to make melt spun nonwoven fabrics, particularly meltblown materials as described in U.S. Pat. No. 5,470,663 and No. 5,401,458, the disclosure of which is herein incorporated by reference.
The present invention contemplates that the provision of a fine denier spunbond layers onto which a meltblown layer is deposited significantly improves the barrier performance of the composite fabric. The fine denier spunbond layer should provide a more uniform interface between the layers during the manufacture of the nonwoven fabric resulting in improved barrier performance in the fabricated article.